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\pagestyle{myheadings} \markright{SMEX Science Outline, Draft 0.01}

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\section*{Crucial Science to be Accomplished by Proposed SMEX Mission}
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\subsection*{Summary Words}

The proposed SMEX mission represents a significant leap forward in
our understanding of the synthesis of galaxies, stars,
planets, and life.  It will survey the Galaxy in the pivotal neutral
and ionic species of C, N, and O that sensitively probe the structure, energy
balance, chemical composition, and evolution of the Galaxy and
external galaxies.  Focused Galactic surveys of interstellar water,
the molecule of life, 
will demonstrate the formation mechanisms, excitation, and critical
role that water is expected to play in the sculpting of star forming
regions and planet-forming disks.  Such large-scale observations are not only
important in their own right, and unobtainable by any other
observatory, but they also dramatically complement, extend and enhance the
interpretation of data from past and contemporary observatories such
as SWAS, SIRTF, SOFIA and Herschel.  That is, this mission makes the
data from existing scientific investments substantially more
meaningful and valuable.

\subsection*{Skeletal Outline of Points}

\begin{itemize}
\item {[CII]} and [NII] Galactic plane survey: 360 degrees of Galactic
  longitudinal coverage, $\pm$2 degrees latitude. 
  \begin{enumerate}
  \item 158 $\mu$m [CII] line emission is the dominant coolant of
  the Galactic interstellar gas,
  stems from CNM, and WNM, a bit from WIM too.  Up to 1\% of the
  far-IR flux comes from this spectral line alone
  \item {[NII]} 205 $\mu$m is unexpectedly 10\% as bright, dominated
  by WIM owing to 14.5~eV ionization energy of N. 
  \item COBE FIRAS lacked spectral and angular resolution to clearly
  demonstrate the physical origins of the mapped emission lines
  \item The origin of these principal C, N, (and O) lines are
  necessary to interpret the structure of the Milky Way, much less the
  far-IR photometry and spectra of Galaxies that SIRTF \& ALMA will yield
  \item Also pivotal to the interpretation of star formation at early
  epochs ([CII] emission shifts from SIRTF to ALMA at high-z)
  \item High angular resolution needed to disentangle multiple sources
  within a single beam, resolve HI structures typically of the order
  of 1 arcminute or smaller... COBE had a 7 degree beam (at 158 $\mu$m?)
  \item High spectral resolution is needed to derive kinematical
  distances and yield a 3D picture of the Galactic C$^+$.  Allows direct
  comparison with existing HI and CO surveys, especially since [CII]
  is expected to have the same complicated structure as HI shows in
  absorption. 
  \end{enumerate}
\item {[OI]} emission at 63 and 145 $\mu$m 
  \begin{enumerate}
    \item Major coolant of warm (dense) gas: n$_{\rm crit} \sim 10^5\
    {\rm cm}^{-3}$ in collisions with H$_2$.  Much lower with just
    e$^-$, on the order of a few $\times 10^2$. 
    \item Along with H$_2$O, [OI] probes a major repository of gas
    phase oxygen not bound up in CO. 
    \item {[OI]} emission largely stems from shocks and the WNM:
    structure of protostellar outflows, surfaces of molecular clouds
    \item 63 $\mu$m emission lies 250K above ground, 145 $\mu$m lies
    350K up.  Measurable emission comes from even cold gas (30-50K),
    but gas must be at or above $n_{\rm crit}$. 
    \item Best strategy would be to make somewhat more targeted
    observations rather than a blind Galactic plane survey. 
  \end{enumerate}
\item H$_2$O in the Galaxy
  \begin{enumerate}
    \item Water is the molecule of life; puzzles regarding its
    abundance and excitation in space remain from SWAS
    \item 179 $\mu$m $2_{12}-1_{01}$ ortho-H$_2$O line is bright, ends in same
    lower state as the SWAS 557~GHz line, excitation energy is about
    75K.  Seen by ISO. 
    \item Only Herschel will be able to observe it; no new surveys planned
    \item High angular and spectral resolution needed to disentangle
    self-absorption from a bright 179 $\mu$m continuum in some regions
    \item Major repository of oxygen in the molecular ISM
    \item Interstellar abundance is low; formed and processed on
    grains in the condensations of protostars.  Released into ISM in
    great abundance when reprocessed.  
    \item Major gas coolant in protostellar disks and envelopes, plays
    a role in sculpting their structure
    \item Intricate connection of star forming environments to Solar
    System comets 
    \item Survey of carbon stars may elucidate the fraction of
    water-carrying cometary systems around other stellar systems
  \end{enumerate}

\item Other lines of interest
  \begin{enumerate}
    \item Many other water lines of interest between 1.4-2.0 THz
    \item Ground state of CH, H$_3$O$^+$; low lying states of OH, CH$^+$
    \item Coarse measurement of continuum 
  \end{enumerate}

\item Connection to other Observatories
  \begin{enumerate}
    \item SIRTF:  MIPS 160 $\mu$m detector expected to be saturated in
    the Galactic plane.  Last useful array is the 70 $\mu$m array. 
    SMEX continuum measurements in the 150-200
    $\mu$m range will add a vital SED point before ground-based
    submillimeter bolometer arrays kick in at 350 $\mu$m, and more like
    870 $\mu$m...
    \item SIRTF:  How does [CII] and [NII] relate to the infrared
    continuum flux and temperature?  How about warm H$_2$ seen by SIRTF/IRS?
    \item AST/RO:  Connection of [CII] to existing CO and [CI]
    surveys 
    \item SIRTF(z$<$1), ALMA (z$>$1):  Measurement of [CII] in other
    galaxies; star formation rates as function of redshift, ISM
    pressure, galaxy physical and chemical evolution, etc... 
    \item SWAS: Extend SWAS maps to cover more of the Galactic plane.
    What is the excitation of o-H$_2$O?  Better estimate of abundances
    and spatial substructure in the water lines, comparison to CO, [CI]...
    \item SOFIA and ALMA: Continuum measurements, [CII] survey, H$_2$O survey
    are needed ``finder charts'' for pointed observations by ALMA and SOFIA
    \item etc...
  \end{enumerate}

\end{itemize}

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